The most damaging thing to CSA war efforts resulting from Gettysburg was Lincoln realizing Meade wasn't the guy he wanted. WHen Meade allowed Lee to slip back into Virginia it was a repeat of the Antietam mistake. The Union had Lee where they wanted him but yet let him escape. Lincoln wanted a general that would go for the kill and not be satisfied with just a victory over Lee. Grant was his guy and it was only a matter of time after that.